1. Field of the Invention
The present invention relates generally to thermal sensors of fluids, such as fluid flow sensors incorporated into a robust package in microstructure form. For convenience sake the term xe2x80x9cflow sensorxe2x80x9d will be used generically hereinafter for such thermal sensors. The reader will appreciate that such sensors may be utilized to measure primary properties such as mass flow rate, temperature, thermal conductivity and specific heat; and that the heat transfers may be generated through forced or natural convection. The invention relates specifically to a cartridge or assembly which exploits the capabilities of a microstructure flow sensor. Even more specifically, a sensor of the Microbrick(trademark) or microfill type is utilized having a central heating element and surrounding sensor arrays which are structurally robust and capable of operating in harsh environments. These Microbrick(trademark) or microfill sensors include through-the-wafer interconnects thus providing very low susceptibility to environmental damage or contamination. The material of the sensor support structure is of thermal conductivity tailored to the application thus producing a more useful and versatile sensor, such as needed for high sensitivity or high mass flux fluid flow measurement or measurements in harsh environments.
2. Description of Related Art
Open microbridge structures such as detailed in U.S. Pat. 5,401,155, to Higashi et al., are well suited for measurements of clean gases, with or without large pressure fluctuations, since the microbridge structure is burst-proof. However, due to the open nature of the microbridge structure, condensates from vapor can be uncontrollably retained in the microbridge structure leading to uncontrolled changes in its thermal response, or output, making the structure susceptible to output error and poor stability.
The typical microbridge structure has a silicon die wire bonded at the top surface to a header, or substrate, carrying further electrical leads and/or electronics. Typically, such wire for the wire bonds would be a one mil gold wire. This wire has a tendency to retain particles suspended in the fluid, retain liquid condensates, increase undesirable turbulence, and shift flow response. Due to its thinness, the wire is also susceptible to damage in a high mass flux environment, such as high rate liquid flow, and upon attempts to clean the sensor.
Membrane-based sensors overcome some of the problems of the microbridge structure because there is no opening exposed to the fluid. More specifically, there is no opening allowing the fluid to enter the underlying structure. However, because the membrane is sealed over an isolation air space and subject to differential pressure induced stress signal errors, membrane based sensors have limited application in high pressure applications. Due to the physical configuration of the membrane, it can deform or burst as pressure differences (on either side of the membrane) increase above 100 PSI (pressure levels that are very possible in high mass flux environments). The heating/sensing elements on the top surface of the membrane sensors are also typically wire bonded to other components, leaving the problems of the wire in the flow path accumulating debris and possibly breaking during cleaning attempts.
While many different materials may be used to make a fluid flow sensor, the choice of material can drastically affect the sensor""s performance. A preferable material making up the sensor substrate would have a relatively low thermal conductivity among other characteristics. This low thermal conductivity is necessary to maintain the sensitivity for the sensor. With this relatively low thermal conductivity, all heating/cooling effects presented to the various sensing elements are caused predominatly by the fluid to be sensed. Stated alternatively, it is important to ensure that heat is not transmitted through the substrate excessively, resulting in signal shorts.
The micromembrane structure discussed above provides a design approach that enables accurate thermal measurements to be made in harsh environments (condensing vapors, with suspended particles, etc.). Specifically, the mass of silicon immediately below the heater/sensing elements is greatly reduced or eliminated, thus limiting potential heat losses. Even in this structure, however, the selection of materials is criticalxe2x80x94low thermal conductivity and appropriate material strength continue to be very important. A disadvantage of this structure is its sensitivity to differential pressure (across its membrane) which induces a stress in the sensing elements and results in uncontrolled output signal changes or errors.
In addition to the above referenced thermal characteristics, it is highly desirable for the overall flow sensor to be chemically inert, corrosion resistant, highly temperature stable, electrically isolated, and bio-compatible. Obviously, many of these characteristics are achieved by proper selection of materials. Further, these desired characteristics are necessary in light of the sensors"" operating environment. The materials chosen must provide for a sensor which is capable of operating in harsh environments.
It would therefore be desirable to develop a flow sensor which is not susceptible to the above referenced problems. Specifically, the sensor would not be affected by vapor accumulation beneath the microbridge, and would not have exposed bonding wire near the heating and sensing elements. The desirable sensor would be structurally robust and thus capable of operating in harsh environments. Further, it would be desirable to develop a flow sensor which is not affected by signal shorts, thus capable of sensing high mass airflows and liquid flows. To accomplish this a desired flow sensor would include a robust substrate or die with relatively low thermal conductivity, high temperature stability, high electrical isolation, corrosion resistance, chemical inertness, and biocompatability. The design of such a structure would enable flow rate and thermal property sensing over wide ranges at high pressure. Further, this capability would provide trouble free operation in hostile environments. The desirable flow sensor and associated housing would also minimize dead volume and promote cost-efficiency, portability, and miniaturization. The sensor would also be adaptable to monitor the flow through a predetermined flow channel attached to the sensor.
The present invention details a microstructure flow sensor having a microsensor die with a Microbrick(trademark) or microfill structure (each having a substantially solid structure beneath the sensing elements) and through-the-wafer electrical interconnections. This structure provides a robust sensor that is operable and accurate in many different applications, including harsh environments.
Additionally, the microstructure flow sensor may be incorporated into an assembly in order to achieve a robust sensing device. The assembly is a robust package, configurable so that it may be operably integrated into a microfluidic cartridge of the type used in lab-on-a-chip systems. The flow sensor in the assembly monitors the controlled flow of fluid in the cartridge and transmits signals though flex circuits indicative of that flow. The integration of the assembly and the cartridge yields the benefits of larger instruments through a smaller device.
The sensor features a flat, passivated, top surface overlying the heater and sensor elements to provide appropriate electrical isolation. Further, a die with through-the-wafer interconnections eliminates the need for bonding wires with their attendant problems as discussed above. In order to withstand a wide range of pressure levels and operate in harsh environments, the die structure is configured to be very robust. The die is made up of materials that have very low thermal conductivity, thus eliminating the possibility of undesired thermal signal shorts. For example, the die may be fabricated using various glass materials, alumina, or combinations of such materials.
The die is attached to a substrate having a suitably matched coefficient of thermal expansion (CTE) by any number of adhesives. Electrical contact is made by thermocompression bonding, solder bumping, conductive adhesives or the like. Preferably the through-the-substrate electrical contacts terminate in the necessary electrically conductive runs for attachment to further electronics of the sensor. This allows for easy interconnection to further devices as described below.
The substrate may further have a passivation layer at the mating surface with the die in order to provide a fluid barrier to the bottom of the die and back fill seals to prevent access to the back-side contacts. Both silicon oxide and silicon nitride layers may be used in the construction of the die. The present invention will benefit the user by trouble free and reliable service in all fluid flow applications as well as being easily fabricated and easily subjected to cleaning maintenance.
The ability to perform high mass flux sensing operations is largely dependent upon the physical characteristics of the sensor. Most importantly, low thermal conductivity of the die substrate is necessary in order to create a sensor capable of operating in these high mass flux sensing situations. By minimizing the thermal conductivity, interference with sensor heating/cooling effects will be minimized and the sensing capabilities are enhanced. Specifically, the characteristics of the die substrate materials will control the proper route of heat transfer, avoiding transfer through the die substrate from the heater to the sensors. Various materials can provide this characteristic. Historically, silicon nitride of a microbridge sensor chip has been used to provide certain levels of thermal conductivity, while also being easily manufactured. However, its fragility prevents is use in harsh environments.
A more optimum material which exhibits the desired characteristic is glass. Glass, however, has not been previously used because it has not been easily micromachined. That is, it is difficult to form the required structures using glass. Another potential substrate material is alumina, which is widely used for electronics packaging and can be machined to serve as substrate with some desirable characteristics. One undesirable feature, however, is its high thermal conductivity, which would severely reduce the sensitivity of the sensor chip.
Recent developments in glass materials, including photosensitive glass and pyrex, have shown that micromachining is possible and extremely effective. Consequently, this material can now provide an alternate die substrate for a micromachined flow and property sensor. The present invention exploits the characteristics of glass (photosensitive glass, fused silica, etc.) or alumina materials to produce a flow and property sensor with optimized physical characteristics. Providing a glass based sensor in a Microbrick(trademark) or microfill structure consequently enables the fabrication of a rugged sensor for sensing liquid properties or high mass flux fluid flow, without pressure-stress-induced error signals.
Due to the recent developments in glass, the use of this material as a die substrate generally reduces the amount of structural machining necessary. More specifically, the substrate can now be fabricated in a Microbrick(trademark) or microfill structure which has a substantially solid structure. In this type of sensor die, the heating and sensing elements are placed directly on the substrate and no further processing or structuring is required beneath those elements. Consequently, the substrate itself is continuous beneath the sensing elements creating a more robust sensor die. The characteristics of the glass substrate material allows this Microbrick(trademark) structure to be effectively used in harsh environments.
Alternatively, the same Microbrick(trademark) structure can be achieved utilizing a plug type configuration. In this approach, a substrate material includes a hole under the heating and sensing elements or opening extending completely therethrough. This hole is then refilled with a filler or plug of appropriate materials creating a microfill structure (i.e. a micro hole filled with solid material). The combination of this substrate and the appropriate filler or plug can effectively tailor the thermal characteristics of the microsensor die. For example, the substrate may be largely fabricated from alumina, and include a glass plug. The heating elements are then placed directly upon this plug element, thus providing the necessary thermal characteristics.
The microstructure flow sensor is configurable such that it may be operably incorporated into a flow sensor assembly in order to achieve a more complete sensing device. Specifically, a flow sensor assembly may include the flow sensor contained within a cooperating sensor housing. The sensor housing defines a sealed flow channel, which directs fluid across the flow sensor, and provides a fluid inlet and a fluid outlet at either end of the flow channel. The sensor die meters the volume of reagents and other fluids flowing through the sealed flow channel. As the configuration of the sealed flow channel is known, this operates as a metering flow channel.
The sensor housing may include two layers; a channeled layer and a cover layer. A precision groove in the channeled layer defines the flow channel, a portion of which is enclosed by the attachment of the cover layer to the channeled layer. The cover layer further defines a window for exposing the flow sensor to transmit signals other devices. The flow sensor is arranged on the channeled layer such that its sensing surface also encloses the remaining portion of the flow channel, creating a sealed metering flow channel. The channeled layer and the cover layer may be attached through a variety of means, including a process whereby an epoxy layer is provided between the layers creating a seal therebetween. The groove in the channeled layer is created with substantial precision such that the resultant flow channel provides the desired flow control. Further, the flow sensor is received in the channeled layer and aligned such that its sensing surface is substantially arranged to provide precise metering of the reagents and other fluids flowing through the flow channel.
The flow sensor assembly is a versatile and robust package, configurable so that it may be operably embedded on a microfluidic cartridge of the type used in lab-on-a-chip systems. The sensor of the assembly monitors the controlled flow of fluid in the cartridge and includes electrical connectors for transmitting signals indicative of that flow.
For example, the flow sensor assembly may enable a micro-cytometry cartridge, designed for counting and classifying cells, especially blood cells. The cartridge may accept a blood sample, and provide a flow path for the blood cells within the cartridge. The cartridge flow path introduces the blood into the fluid inlet of the sensor assembly and, after metering the blood flow rate across the sensor, receives the cells from fluid outlet. The micro-cytometry cartridge may further implement multiple sensor assemblies, each metering the flow rate of blood and other reagents at distinct points in the cartridge path.
This integrated structure brings the capabilities of larger and costlier instruments to smaller devices and provides many benefits. The reduced size of the sensor assembly and the cartridge promotes cost reduction and minimizes dead volume flow. The small size and light weight design of the sensor and cartridge increase its portability and self-containment advantages. Additionally, a sensor assembly embedded on a cartridge may be reused on another cartridge, thereby sparing the expense of a new sensor after each use and at the same time benefiting from the functionality and capabilities provided by the cartridge. Depending on the nature of the cartridge, however, it may be preferable to dispose of both sensor assembly and the cartridge after use.